Added support to run Panbuild on Python 3.7

[Zildj1an]

Made Panbuild runnable on Python 3:

• filter() doesn't return a list in the new Python, but a iterable filter object (source). If needed, it is converted to a list.
• Using yaml.safe_load() instead of yaml.load() avoids new warnings that prevent execution.

Tested and working for Python 3.7.3